Swirlers are used as mixing devices in various technical applications. Optimization of swirlers aims at reducing the energy required to obtain a specified degree of homogeneity of a mixture. In continuous flow mixing the pressure drop over a mixing device is a measure for the required energy. Further, the time and space required to obtain the specified degree of homogeneity are important parameters for the evaluation of mixing devices or mixing elements. Swirlers are typically used for mixing of two or more continuous fluid streams. Axial swirlers are most commonly used as premixers in gas turbine combustors.
A socalled swirl number sn characterizes the swirl strength of an axial swirler. The swirl number is defined as the ratio between the axial flux of azimuthal momentum and the axial flux of axial momentum multiplied by the swirler radius. The swirl number is an indication of the intensity of swirl in the annular flow induced by the swirler.
With conventional axial swirlers a required recirculation zone is formed only if the swirl number is equal to or larger than about 0.4. The larger is the swirl number, the larger is the pressure drop entailing an unfavorable energy loss.
Conventional Axial swirlers with upstream fuel injection typically produce unfavorable wakes and recirculation regions at the fuel injection location.
Axial swirlers with fuel injection via the swirlers, so-called swozzles, require for good air-fuel mixing swirl numbers that are so high that it is difficult to avoid wakes and flow-separation regions at the fuel injection location and trailing edge.
Dual annular counter-rotating axial swirlers result in a radially varying velocity distribution that is unfavorable for the combustion process for e.g. poor flame holding resistance and combustion dynamics.